1. Technical Field
This invention relates to polyethylene prepared using Ziegler-Natta catalyst systems. This invention particularly relates to polyethylene prepared using mixed Ziegler-Natta catalyst systems.
2. Background of the Art
Olefins, also called alkenes, are unsaturated hydrocarbons whose molecules contain one or more pairs of carbon atoms linked together by a double bond. When subjected to a polymerization process, olefins are converted to polyolefins, such as polyethylene and polypropylene.
Polyethylene polymers are finding increasing numbers of applications in the thermoplastics industry. These polymers offer relatively good strength and other performance properties when used in applications such as, for example, films and blow molding applications such as for preparation of bottles and the like. For these applications the polyethylene is desirably processable over a wide variety of processing conditions. It is also desirable that processing be enabled for a variety of types of processing equipment. Finally, it is desirable that the final polyethylene product exhibits good physical properties and commercially desirable appearance.
Ziegler-Natta type polyolefin catalysts, their general methods of making, and subsequent use, are known in the polymerization art. While much is known about Ziegler-Natta type catalysts, there is a constant search for improvements in their polymer yield, catalyst life, catalyst activity, amenability to use in large scale production processes, and in their ability to produce polyolefins having certain properties such as particle morphology.
Ziegler-Natta catalysts comprise a transition metal complex generally represented by the formula:MRx where M is a transition metal, R is a halogen or a hydrocarboxyl, and x is the valence of the transition metal. Typically, M is a group IVB metal such as titanium, chromium, or vanadium, and R is chlorine, bromine, or an alkoxy group.
The properties of the polymerization catalyst may affect the properties of the polymer formed using the catalyst. For example, polymer morphology typically depends upon catalyst morphology. Acceptable polymer morphology differs for each class of production process (e.g., slurry loop, bimodal, gas phase, etc.), but typically includes uniformity of particle size and shape and an acceptable bulk density.